Recently, processing techniques to encode and decode a moving picture have been rapidly developed. In a digital television receiving digital broadcast and a record/play apparatus having a recording medium such as a HDD (Hard Disk Drive) or a DVD (Digital Versatile Disk), MPEG-2 (Moving Picture Experts Group) format is mainly used.
The MPEG-Encoding (represented as MPEG-2 format) is a coding method using MC (Motion Compensation Prediction) and DCT (Discrete Cosine Transformation). In case of encoding at a low bit rate, “block noise” generated around a block boundary and “ringing noise” generated around an edge boundary are often perceived.
In JP-A (Kokai) No. 2003-179921, a method for reducing such noise (caused by the MPEG-Encoding) by using coding information is disclosed. In this method, by referring to coding information (quantization information, DCT coefficient information, motion vector information) obtained by decoding MPEG coding data, a kind and an intensity of a filter to reduce the noise of decoded image are switched.
As the kind of the filter, a block noise reduction filter, a mosquito noise reduction filter, and a Gaussian filter are applied. In this case, mosquito noise means ringing noise.
In JP No. 3464908, a block noise reduction filter (de-block filter) is disclosed. Furthermore, in JP-A (Kokai) No. 2000-102020, a ringing noise reduction filter (de-ringing filter) is disclosed.
In JP-A No. 2003-179921, in order to reduce processing quantity, an adaptive filter is selected by referring to coding information only. However, if decoded image quality estimated by the coding information is largely different from actual decoded image quality, the noise cannot be sufficiently reduced. Furthermore, a signal to be originally existent is erroneously reduced.